Laser imaging devices, including laser printers and copiers, are used in a wide variety of computer systems. Efforts are continually being made to increase the printing/copying speed of such devices, while at the same time maintaining quality and reducing their cost. Laser printers are designed to produce an image on paper or other print material by placing overlapping dots at virtual positions defined by a digital raster. These virtual positions in the raster are known as “pixels”, which is short for “picture elements”. In laser printers, the image is typically first produced on a charged photoconductive material by scanning the photoconductor with a laser beam in a sequence of scan lines. A laser printer may scan the surface of the photoconductive material with a laser in the sequence of scan lines using a rotating polygonal mirror. As a line is scanned, or written, the laser beam is energized during the extent of each pixel that corresponds to a dot in the image. When the laser beam is energized, the portion of the photoconductor upon which the laser energy impinges is discharged, thereby producing a charge-pattern copy of the desired image on the photoconductor. The printed output is obtained by attracting toner to the charge-pattern and transferring the developed image to paper or other print material.
In order to increase the operating speed of such a laser imaging device, laser printers may use multiple laser beams to concurrently scan multiple scan lines that are vertically offset from one another by an incremental amount. Such imaging devices may be referred to as multiple-beam engines. By concurrently scanning multiple laser beams, the time required to scan the photoconductive material is decreased, which results in an increased exposure and print speed. However, for a multiple beam laser printer, good horizontal alignment of the scan lines produced by the individual lasers can help to produce a high quality image. Unfortunately, due to slight variations in mechanical tolerances of the laser units, photoconductor, and the length of video paths of printed circuit boards, there is almost inherently a slight horizontal mis-alignment between scan lines. For example, in a laser printer where the lasers scan from left to right across the photoconductive material, one laser may begin to write pixel data at a first point, while a second laser may begin to write pixel data at a second point that is to the left of the first point. The result is that the scan lines are not horizontally aligned and image quality is reduced. Misalignment can cause a variety of print artifacts, such as lines that do not appear straight.